SUMMARY: The majority of HNSCC patients die from local recurrences and/or metastasis that can occur early, or many years after primary tumor removal. This can be due to disseminated tumor cells (DTCs) remaining dormant in patients. DTCs can be detected in the bone marrow (BM), but rarely give rise to metastasis, suggesting a dormant state in this site. We discovered that dormancy of HNSCC DTCs precedes metastasis and that dormant DTCs are governed by unique chromatin states and microenvironmental cues that can be derived from the vascular niche. Through loss-of-function screens, we also identified chromatin remodelers that enforce tumor cell dormancy. Further, using information we derived from senescence models, we determined that dormant DTCs express chromatin regulators and secreted factors found in senescent cells. We hypothesize that specific microenvironmental cues and epigenetic regulators induce and maintain a strong, yet reversible, growth arrest program during the dormant state of DTCs. We found that the quiescence and differentiation signals, retinoic acid (atRA), TGF?2 and BMP7 can induce dormancy and metastasis suppression. Dormant cancer cells also display a global repressive chromatin state and upregulation of specific transcription factors (NR2F1) and histone variants (macroH2A, clipped H3.3), found upregulated during differentiation, senescence and metastasis suppression. In addition, CRISPR-Cas9 screens identified the chromatin remodeler ARID1A as an enforcer of dormancy. We propose that microenvironmental and epigenetic regulators converge to induce and maintain dormancy. We will test this hypothesis by A) investigating the role of histone variant macroH2A associated with quiescence and senescence programs in DTC dormancy induction, B) deciphering how ARID1A enforces dormancy and C) by identifying chromatin permissive states for dormancy induction by microenviromental signals. MPI SYNERGY: The Aguirre-Ghiso and Bernstein labs along with our collaborators have a productive history of publications and active collaborations. This team possesses specialized expertise in dormancy, metastasis, and chromatin biology that has delivered breakthroughs in the cancer dormancy and metastasis fields. The Condeelis team will provide the most advanced intravital imaging to dissect the epigenetic and microenvironmental regulation of DTC dormancy onset and survival at single cell resolution. Importantly, the Klein and Stoecklein labs will allow validation of our findings in human DTC samples. This provides an unsurpassed opportunity to dissect the microenvironmental and epigenetic regulation of single DTC dormancy.